Plants
Human beings, other animals and plants are the living organisms that exist on earth. Plants are the only organisms able to sustain themselves by producing their own food. In turn, they provide food for animals and humans through the food chain. Almost all plants have one common characteristic making them different from animals. Plants, such a trees, flowers, fruits and vegetables, produce chlorophyll, a substance that allows them to convert solar energy into food. People, as well as animals, on the other hand, obtain their nutrition either by consuming plants or by consuming other animals. Humans eat both meat and plants. Some plants, however, are not able to use sunlight and soil to produce their own source of energy. For example, molds are '''parasites '''obtaining their energy directly from the plant or animal they live on — '''their host. '''Furthermore, plants such as molds and ferns do not reproduce through seeds; they reproduce by creating spores. Flowering plants grow from seeds. A sprouting seed must absorb water before it will start to grow. It must also have soil firmly packed around it and have warmth from the sun. Inside the seed is a tiny '''embryo, '''surrounded by stored food. When the embryo starts growing, roots grow downward and a stem grows upward. Once the stem breaks through the surface of the soil into the sunlight, the first two true leaves form and the plant begins to make its own food. When plants have water, sunlight and the proper minerals in the soil, they grow, manufacture food and give off oxygen. Many plants do not have to grow from seeds. A potato, for example, is not a seed, but it can reproduce itself by growing roots from a specialized part of the potato. Other plants (some cacti) can begin to grow if a small piece of the plant falls on soil. After growing roots, if then the potato is anchored in the soil, it will receive nutrients and produce more potatoes. Some plants send out underground rhizomes that send up new plants periodically. Non-flowering plants grow from spores. Like a seed, a spore develops into an embryo. Unlike a seed, the spore does not contain food to enable the embryo to grow. The plant that develops must get its food from a host. Molds are plants that grow on their hosts, taking nutrients directly from them. Molds do not require light or soil since they don’t produce their own food the way other plants do, but they do require moisture. The food molds eat are the bread, jelly, cheese, fruit, flowering plants, rooting logs and leaves, etc. that they live on. These foods are called “hosts”. Structure Many plants do not have to grow from seeds. A potato, for example, is not a seed, but it can reproduce itself by growing roots from a specialized part of the potato. Other plants (some cacti) can begin to grow if a small piece of the plant falls on soil. After growing roots, if then the potato is anchored in the soil, it will receive nutrients and produce more potatoes. Some plants send out underground rhizomes that send up new plants periodically. Plant cells are formed at meristems, and then develop into cell types which are grouped into tissues. Plants have only three tissue types: 1) system Dermal; 2) system Ground; and 3) system Vascular. Dermal tissue covers the outer surface of herbaceous plants. Dermal tissue is composed of epidermal cells, closely packed cells that secrete a waxy cuticle that aids in the prevention of water loss. The ground tissue comprises the bulk of the primary plant body. Parenchyma, collenchyma, and sclerenchyma cells are common in the ground tissue. Vascular tissue transports food, water, hormones and minerals within the plant. Vascular tissue includes xylem, phloem, parenchyma, and cambium cells. Plant cell types rise by mitosis from a tissue meristem. A meristem may be defined as a region of localized mitosis. Meristems may be at the tip of the shoot or root (a type known as the meristem apical meristem) or lateral, occurring in cylinders extending nearly the length of the plant. A cambium is a lateral meristem that produces (usually) growth secondary growth. Secondary growth produces both wood and cork (although from separate (lateral) meristems secondary meristems). Parenchyma Plant cell types rise by mitosis from a tissue meristem. A meristem may be defined as a region of localized mitosis. Meristems may be at the tip of the shoot or root (a type known as the meristem apical meristem) or lateral, occurring in cylinders extending nearly the length of the plant. A cambium is a lateral meristem that produces (usually) growth secondary growth. Secondary growth produces both wood and cork (although from separate (lateral) meristems secondary meristems). http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookglossC.html#collenchymathumb|Collenchyma cells. Note the thick walls on the collenchyma cells occurring at the edges of the Medicago stem cross section.|link=http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookglossC.html#collenchyma Collenchyma Collenchyma cells support the plant. These cells are characterized by thickenings of the wall, they are alive at maturity. They tend to occur as part of vascular bundles or on the corners of angular stems. In many prepared slides they stain red. http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookglossS.html#sclerenchymathumb|Sclerenchyma cells|link=http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookglossS.html#sclerenchyma Sclerenchyma Sclerenchyma cells support the plant. They often occur as bundle cap fibers. Sclerenchyma cells are characterized by thickenings in their secondary walls. They are dead at maturity. They, like collenchyma, stain red in many commonly used prepared slides. A common type of schlerenchyma cell is the fiber. Some sclerenchyma cells occur in the fruits of pear trees. These cells (sclereids or stone cells) give pears their gritty texture. Xylem Xylem is a term applied to woody (lignin-impregnated) walls of certain cells of plants. Xylem cells tend to conduct water and minerals from roots to leaves. While parenchyma cells do occur within what is commonly termed the "xylem" the more identifiable cells, tracheids and elements vessel elements, tend to stain red with Safranin-O. Tracheids are the more primitive of the two cell types, occurring in the earliest vascular plants. Tracheids are long and tapered, with angled end-plates that connect cell to cell. Vessel elements are shorter, much wider, and lack end plates. They occur only in angiosperms, the most recently evolved large group of plants. Tracheids, longer, and narrower than most vessels, appear first in the fossil record. Vessels occur later. Tracheids have obliquely-angled endwalls cut across by bars. The evolutionary trend in vessels is for shorter cells, with no bars on the endwalls. http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookglossPQ.html#phloemthumb|Phloem cells|link=http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookglossPQ.html#phloem Phloem Cells Phloem cells conduct food from leaves to rest of the plant. They are alive at maturity and tend to stain green (with the stain fast green). Phloem cells are usually located outside the xylem. The two most common cells in the phloem are the cells companion cells and cells sieve cells. Companion cells retain their nucleus and control the adjacent sieve cells. Dissolved food, as sucrose, flows through the sieve cells. Epidermal Cells Epidermis The epidermal tissue functions in prevention of water loss and acts as a barrier to fungi and other invaders. Thus, epidermal cells are closely packed, with little intercellular space. To further cut down on water loss, many plants have a waxy cuticle layer deposited on top of the epidermal cells. Guard Cells To facilitate gas exchange between the inner parts of leaves, stems, and fruits, plants have a series of openings known as stomata (singular stoma). Obviously these openings would allow gas exchange, but at a cost of water loss. cells Guard cells are bean-shaped cells covering the stomata opening. They regulate exchange of water vapor, oxygen and carbon dioxide through the stoma.